Optical waveguide device

ABSTRACT

An optical waveguide device having multiple functions or high performance, to improve the productivity of products, and to provide an optical waveguide device capable of suppressing deterioration of an operating characteristic of the optical waveguide device, including a thin plate  1  having a thickness of 20 μm or less, and at least an optical waveguide  2  formed in the thin plate. The thin plate is bonded and fixed to a supporting substrate  5  with an adhesive  4  interposed therebetween, and a film having a higher refractive index than the thin plate is provided on a surface of the thin plate bonded and fixed to the supporting substrate so as to be in contact with a part of the optical waveguide.

TECHNICAL FIELD

This application is a divisional of application Ser. No. 12/225,783filed Dec. 12, 2008, which was a §371 of PCT/JP2007/054233 filed Mar. 6,2007, claiming priority from Japan Pat. App. No. 2006-092293 filed Mar.29, 2006, all of which are incorporated herein by reference. The presentinvention relates to an optical waveguide device, and more particularly,to an optical waveguide device which uses a thin plate having athickness of 20 μm or less and has at least an optical waveguide formedin the thin plate.

BACKGROUND ART

In an optical communication field or an optical measurement field, anoptical waveguide device in which an optical waveguide or modulationelectrodes are formed on a substrate having an electro-optical effect orthe like has been often used in the related art.

In particular, since the amount of transmitted information tends toincrease with the development of multimedia, it is necessary to widen aband of a light modulation frequency. In order to realize this, anexternal modulation method using an LN modulator or the like has beenproposed. However, in order to widen the band of the LN modulator, it isnecessary to realize speed matching between a light wave and amicrowave, which is a modulated signal, and to reduce a driving voltage.

In order to solve the problems, it is known that a condition of speedmatching between a microwave and a light wave is satisfied, and at thesame time, a driving voltage is reduced by making a substrate thinnerthan in the related art.

In the following Patent Document 1 or 2, an effective refractive indexof a microwave is reduced by providing an optical waveguide andmodulation electrodes on a thin substrate (hereinafter, referred to as a‘first substrate’) having a thickness of 30 μm or less and bondinganother substrate (hereinafter, referred to as a ‘second substrate’)having a dielectric constant lower than the first substrate to the firstsubstrate, such that the speed matching between the microwave and thelight wave is realized and the mechanical strength of the substrate ismaintained.

-   Patent Document 1: JP-A-64-18121-   Patent Document 2: JP-A-2003-215519

In Patent Document 1 or 2, LiNbO₃ (hereinafter, referred to as ‘LN’) ismainly used for the first substrate and a material having a lowerdielectric constant than LN, such as quartz, glass, and alumina, ismainly used for the second substrate. In the combination of thesematerials, temperature drift or DC drift according to a temperaturechange occurs due to a difference between coefficients of linearexpansion. In order to eliminate such problem, Patent Document 2discloses that the first substrate and the second substrate are bondedto each other using an adhesive having a coefficient of linear expansionclose to the first substrate.

On the other hand, use of the optical waveguide device is not limited tothe light modulator described above. The optical waveguide device isrequested to have multiple functions, for example, to be used as adiffraction grating or a variable wavelength filter. In addition, theoptical waveguide device is also requested to have high performance, forexample, to monitor a control state of the optical waveguide device oran optical integrated circuit, in which a plurality of optical waveguidedevices are provided.

For example, in a variable wavelength filter, not only an opticalwaveguide or an electrode is provided in an optical waveguide device,but also a titanium oxide layer and unevenness need to be formed on theoptical waveguide in order to form the refractive index distributionwhich periodically changes along the optical waveguide as disclosed inPatent Document 3 or 4.

-   Patent Document 3: JP-A-5-88123-   Patent Document 4: JP-A-5-264809

Moreover, in the optical integrated circuit, optical waveguides need tobe bent near an end of a substrate, and accordingly, a reflecting means,such as a green lens, needs to be arranged on the end surface of thesubstrate. In addition, in order to monitor a lightwave propagatingthrough the optical waveguide device, it is necessary to provide anoptical waveguide or a directional coupler for monitoring.

Thus, in order to realize an optical waveguide device having multiplefunctions or high performance, the number of optical waveguides orelectrodes provided in the optical waveguide device increases andvarious kinds of films, unevenness shape, and the like increase. As aresult, the optical waveguide device itself becomes complicated.Furthermore, if the members described above are formed on the thin platedescribed above, the thin plate is easily damaged due to a differencebetween coefficients of thermal expansion of the thin plate and each ofthe members or due to mechanical or thermal stress distortion caused bythe complicated shape of the thin plate. As a result, problems thatyield of products decreases, unnecessary stress is applied to the thinplate and an operating characteristic of the optical waveguide devicedeteriorates, and the like occur.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In order to solve the problems described above, it is an object of theinvention to realize an optical waveguide device having multiplefunctions or high performance, to improve the productivity of products,and to provide an optical waveguide device capable of suppressingdeterioration of an operating characteristic of the optical waveguidedevice.

Means for Solving the Problems

In order to solve the above problems, according to a first aspect of theinvention, an optical waveguide device includes: a thin plate having athickness of 20 μm or less; and at least an optical waveguide formed inthe thin plate. The thin plate is bonded and fixed to a supportingsubstrate with an adhesive interposed therebetween, and a film having ahigher refractive index than the thin plate and the adhesive is providedon a surface of the thin plate bonded and fixed to the supportingsubstrate so as to be in contact with or close to at least a part of theoptical waveguide.

In addition, in the invention, ‘on a surface’ means not only a frontsurface of the thin plate, but also a front surface after forming abuffer layer, such as an SiO₂ film, on the thin plate is included.

Furthermore, in the invention, ‘being in contact’ means not only a stateof direct contact, but also indirect contact realized through a bufferlayer or the like is included.

According to a second aspect of the invention, in the optical waveguidedevice according to the first aspect, the thin plate is formed of amaterial having a nonlinear optical effect or an electro-optical effect.

According to a third aspect of the invention, in the optical waveguidedevice according to the first or second aspect, an electrode or a heaterfor controlling the intensity, wave front, or a phase of lightpropagating through the optical waveguide is formed on the other surfaceof the thin plate not bonded and fixed to the supporting substrate.

According to a fourth aspect of the invention, in the optical waveguidedevice according to any one of the first to third aspects, the filmhaving a high refractive index is formed so as to be periodicallyseparated in the guiding direction of the optical waveguide.

According to a fifth aspect of the invention, in the optical waveguidedevice according to the third aspect, a light detection means fordetecting either scattered light from the optical waveguide formed withthe film having a high refractive index or led-out light obtained bycausing a part of light propagating through the optical waveguide to beled out by means of the film having a high refractive index and acontrol means for controlling a voltage or a current applied to theelectrode on the basis of the amount of light detected by the lightdetection means are further included, and the intensity, the wave front,or the phase of light propagating through the optical waveguide iscontrolled.

According to a sixth aspect of the invention, in the optical waveguidedevice according to any one of the first to third aspects, the filmhaving a high refractive index is formed in a strip line shape.

Effects of the Invention

According to the first aspect of the invention, the optical waveguidedevice includes the thin plate having a thickness of 20 μm or less andat least an optical waveguide formed in the thin plate, the thin plateis bonded and fixed to a supporting substrate with an adhesiveinterposed therebetween, and a film having a higher refractive index(hereinafter, referred to as a ‘high refractive index film’) than thethin plate and the adhesive is provided on a surface of the thin platebonded and fixed to the supporting substrate so as to be in contact withor close to at least a part of the optical waveguide. Accordingly, itbecomes possible to configure various kinds of members, such as anoptical waveguide and a diffraction grating, using the high refractiveindex film, and it becomes also possible to arrange various kinds ofmembers on both surfaces of the thin plate. As a result, an opticalwaveguide device having multiple functions and high performance can beeasily realized. In addition, since it is possible to suppress localconcentration of mechanical or thermal stress in the thin plate, it ispossible to prevent the thin plate from being damaged or an operatingcharacteristic of the optical waveguide device from changing. As aresult, it becomes possible to provide an optical waveguide device whichis excellent in terms of productivity and has an operatingcharacteristic that is stabilized.

According to the second aspect of the invention, since the thin plate isformed of a material having a nonlinear optical effect or anelectro-optical effect, it becomes possible to provide various kinds ofoptical waveguide devices, such as an optical harmonic generator, alight modulator, a variable wavelength filter, or an optical switch.

According to the third aspect of the invention, since the electrode orthe heater for controlling the intensity, the wave front, or the phaseof light propagating through the optical waveguide is formed on theother surface of the thin plate not bonded and fixed to the supportingsubstrate, it becomes possible to provide various kinds of opticalwaveguide devices, such as an optical harmonic generator, a lightmodulator, a variable wavelength filter, or an optical switch.

According to the fourth aspect of the invention, since the highrefractive index film is formed so as to be periodically separated inthe guiding direction of the optical waveguide, the high refractiveindex film can function as a diffraction grating. As a result, itbecomes possible to obtain an optical waveguide device, such as awavelength filter.

According to the fifth aspect of the invention, the light detectionmeans for detecting either the scattered light from the opticalwaveguide formed with the high refractive index film or the led-outlight obtained by causing a part of light propagating through theoptical waveguide to be led out by means of the high refractive indexfilm and a control means for controlling a voltage or a current appliedto the electrode on the basis of the amount of light detected by thelight detection means are further included, and the intensity, the wavefront, or the phase of light propagating through the optical waveguideis controlled. Accordingly, it becomes possible to provide ahigh-performance optical waveguide device capable of monitoring a lightwave propagating inside the optical waveguide device and controlling adriving state of the optical waveguide device.

According to the sixth aspect of the invention, since the highrefractive index film is formed in the strip line shape, the highrefractive index film can be used as an optical waveguide. Thus, sincethe high refractive index film can be used as a bent optical waveguideor the like, it becomes possible to provide a high-performance opticalwaveguide device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an optical waveguidedevice according to an embodiment of the invention.

FIG. 2 is a cross-sectional view illustrating an optical waveguidedevice according to a first embodiment of the invention.

FIG. 3 is a cross-sectional view illustrating an optical waveguidedevice according to a second embodiment of the invention.

FIG. 4 is a perspective view illustrating an optical waveguide deviceaccording to a third embodiment of the invention.

FIG. 5 is a perspective view illustrating an optical waveguide deviceaccording to a fourth embodiment of the invention.

REFERENCE NUMERALS

-   1: thin plate-   2: optical waveguide-   3: modulation electrode-   4: bonding layer-   5: supporting substrate-   6: high refractive index film

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the invention will be described in detail using preferredexamples.

FIG. 1 shows an optical waveguide device according to an embodiment ofthe invention. FIG. 1 is a cross-sectional view illustrating a lightmodulation device that includes a thin plate 1, which is formed of amaterial having an electro-optical effect or a nonlinear optical effectand has a thickness of 20 μm or less, an optical waveguide 2 formed on afront surface of the thin plate, and a control electrode 3 which isformed on the front surface of the thin plate and is used to controllight passing through the optical waveguide. In addition, the opticalwaveguide 2 may be formed on a bottom surface of the thin plate 1, and aheater may be used instead of the control electrode 3 depending on thetype of the optical waveguide device.

A supporting substrate 5 is bonded to the thin plate 1 with an adhesive4 interposed therebetween.

As a method of forming the optical waveguide 2, the optical waveguide 2can be formed by diffusing, for example, Ti on a substrate surface usinga thermal diffusion method, a proton exchange method, or the like. Inaddition, as disclosed in Patent Document 5, the optical waveguide mayalso be configured by forming a ridge on a front surface of the thinplate 1 according to the shape of the optical waveguide.

A signal electrode or a grounding electrode or the control electrode 3,such as a DC electrode, may be formed by formation of an electrodepattern using Ti and Au and a metal plating method, for example.Moreover, if necessary, it may also be possible to provide a bufferlayer (not shown) formed of a dielectric material, such as SiO₂, on asubstrate surface after forming an optical waveguide and to form acontrol electrode on the buffer layer.

-   Patent Document 5: JP-A-6-289341

Materials having an electro-optical effect include a lithium niobate, alithium tantalate, a PLZT (lead lanthanum zirconate titanate), a quartz,and a combination thereof, for example. In particular, lithium niobate(LN) crystal having a high electro-optical effect is preferably used.

In addition, materials having a nonlinear optical effect include an ADP(ammonium dihydrogen phosphate), Ba₂NaNb₅O₁₅ (barium-sodium niobate),CdSe (cadmium selenide), a KDP (potassium dihydrogen phosphate), LiNbO₃(lithium niobate), Se (selenium), and Te (tellurium), for example.

In a method of manufacturing the thin plate 1 including a lightmodulation device, the optical waveguide described above is formed on asubstrate having a thickness of hundreds of micrometers and a bottomsurface of the substrate is grinded, thereby forming a thin plate havinga thickness of 20 μm or less. Then, a modulation electrode is formed onthe front surface of the thin plate. In addition, the bottom surface ofthe substrate may be grinded after forming the optical waveguide, themodulation electrode, or the like. In addition, if a thermal impact whenforming an optical waveguide or a mechanical impact caused by handlingof a thin film in various kinds of processing is applied, there is arisk that a thin plate will be damaged. For this reason, it ispreferable to perform processing in which such thermal or mechanicalimpact is easily applied before grinding a substrate to make thesubstrate have a small thickness.

Various kinds of materials may be used for the supporting substrate 5.For example, in addition to the same material as the thin plate,materials having lower dielectric constants than the thin plate, such asa quartz, glass, and an alumina, may be used, or materials havingdifferent crystal orientation from the thin plate may also be used asdisclosed in Patent Document 4. In this case, it is preferable to selecta material having the same coefficient of linear expansion as the thinplate in order to stabilize a modulation characteristic of a lightmodulation device with respect to a temperature change. If it isdifficult to select the same material, a material having the samecoefficient of linear expansion as the thin plate is selected for anadhesive used to bond the thin plate and the supporting substrate asdisclosed in Patent Document 2.

In order to bond the thin plate 1 and the supporting substrate 5 to eachother, various kinds of adhesive materials, such as an epoxy-basedadhesive, a thermosetting adhesive, an ultraviolet curable adhesive,solder glass, a thermosetting resin adhesive sheet, a light curableresin adhesive sheet, or a light-viscosity resin adhesive sheet may beused as the bonding layer 4. A refractive index of the bonding layer 4is preferably lower than that of the thin plate 1, especially, that ofthe optical waveguide 2 in order to suppress a propagation loss of theoptical waveguide 2.

As shown in FIG. 1, the optical waveguide device according to theinvention is characterized in that a film (high refractive index film)6, which has a refractive index higher than those of the thin plate 1and an adhesive, is provided on a surface (lower surface of the thinplate 1 shown in FIG. 1) of the thin plate 1 bonded and fixed to thesupporting substrate 5 so as to be in contact with or close to at leasta part of the optical waveguide 2.

Various kinds of shapes or materials may be used for the high refractiveindex film according to functions and characteristics of the opticalwaveguide device. Hereinafter, an embodiment in which a high refractiveindex film is used will be described.

FIG. 2 is a cross-sectional view illustrating an optical waveguidedevice according to a first embodiment of the invention and shows anexample of the cross-sectional shape of the optical waveguide devicetaken along a dashed-dotted line A of FIG. 1.

High refractive index films 6 are formed on a lower surface of the thinplate 1 at a predetermined period (distance s) therebetween. Due to theshape of such high refractive index film, the high refractive index filmfunctions as a diffraction grating with respect to the optical waveguide2, and the optical waveguide device can operate as a wavelength filter.

A wavelength λ to be selected is determined by the relationship of λ=2ns/m (where ‘n’ is an effective refractive index of a light wave, ‘s’ isan array period of high refractive index films, and ‘m’ is a naturalnumber). In addition, it is possible to change the wavelength to beselected by changing the intensity of an electric field applied by theelectrode 3. In this case, the optical waveguide device functions as avariable wavelength filter.

In addition, interaction between the optical waveguide 2 and the highrefractive index film 6 may be reinforced by forming the opticalwaveguide 2 on a bottom surface of the thin plate 1 such that theoptical waveguide 2 comes into direct contact with the high refractiveindex film. Moreover, if necessary, it is also possible to form a thinbuffer layer between the high refractive index film 6 and the opticalwaveguide 2 formed on the bottom surface of the thin plate 1.

Furthermore, when the thickness of the thin plate 1 is set to 20 μm orless, especially, to 10 μm or less, the mode diameter of a light wavepropagating through the optical waveguide 2 is almost equal to thethickness of the thin plate 1. Accordingly, even if the opticalwaveguide is formed on either a front surface or a bottom surface of thethin plate 1, the influence of the contain electrode 3 or the highrefractive index film 6 on a light wave propagating through the opticalwaveguide is equal.

FIG. 3 is a cross-sectional view illustrating an optical waveguidedevice according to a second embodiment of the invention and shows anexample of the cross-sectional shape of the optical waveguide devicetaken along the dashed-dotted line A of FIG. 1.

The high refractive index film 6 is arranged so as to be in contact withor close to a part of the optical waveguide 2, and a part of electricwaves 10 propagating through the optical waveguide 2 is led to theoutside of the optical waveguide device as indicated by a dotted line12. A part of light waves is led to the outside, but the remaining lightwaves 11 keep propagating through the optical waveguide. Therefore, astate of light waves propagating inside the optical waveguide device canbe easily monitored and determined by detecting the led-out light 12with a photodetector 13.

The shape of the high refractive index film 6 shown in FIG. 3 is notlimited to only the thing of a rectangular parallelepiped. For example,even if the shape is an irregular or indefinite one, a high refractiveindex film exists near the optical waveguide 2, and accordingly,electric waves that propagate are disordered and a part of light wavesis output as scattered light.

According to a result detected by the photodetector 13, a voltage or acurrent applied to the control electrode 3 is controlled by a controlcircuit. As a result, the intensity, wave front, or a phase of lightpropagating inside the optical waveguide device is maintained in apredetermined state.

FIG. 4 illustrates an optical waveguide device according to a thirdembodiment of the invention.

In FIG. 4, the supporting substrate 5 and the bonding layer 4 includedin the optical waveguide device are omitted in order to clearly show theappearance of the high refractive index film formed on a surface of thethin plate 1. In addition, the bonding layer and the supportingsubstrate are arranged at an upper side of the thin plate 1 shown inFIG. 4.

The optical waveguide 2 is formed on a surface of the thin plate 1, andthe control electrode 3 is formed on the other surface of the thin plate1. The high refractive index film 6 that is in contact with the opticalwaveguide 2 and has a strip line shape is formed in a shape shown inFIG. 4. The high refractive index film 6 becomes thick along the opticalwaveguide 2 and is bent in the middle so as to be separated from theoptical waveguide 2.

By providing the high refractive index film 6, a part of light waves 20incident on the optical waveguide 2 moves from the optical waveguide 2to the high refractive index film 6 and is then output as led-out light22 from the other end of the high refractive index film 6 to the outsideof the optical waveguide device. Since a material having a lowrefractive index, which serves as a bonding layer, is filled around thehigh refractive index film 6, the optical waveguide having the highrefractive index film 6 as a core is formed.

As described above, the led-out light 22 is detected by thephotodetector 13 and is used to monitor a state of the optical waveguidedevice, for example. The remaining light waves that were not led out tothe high refractive index film keep propagating through the opticalwaveguide 2 and are output as output light 21 to the optical waveguidedevice.

FIG. 5 illustrates an optical waveguide device according to a fourthembodiment of the invention.

In FIG. 5, the electrode 3, the supporting substrate 5, and the bondinglayer 4 included in the optical waveguide device are omitted in order toclearly show the appearance of the high refractive index film formed ona surface of the thin plate 1. In addition, the bonding layer and thesupporting substrate are arranged at an upper side of the thin plate 1shown in FIG. 4, and the electrode is arranged at a lower side of thethin plate 1.

Two Mach-Zehnder optical waveguides 2-1 and 2-2 are formed in the thinplate 1. Here, the optical waveguide 2-1 and the optical waveguide 2-2are not connected continuously to each other.

On a surface of the thin plate 1, the high refractive index film 6 thatis in contact with the optical waveguides 2-1 and 2-2 and has a stripline shape is formed in a shape shown in FIG. 5. The high refractiveindex film 6 becomes thick along the optical waveguides 2-1 and 2-2 andis bent such that the optical waveguides 2-1 and 2-2 are connected toeach other.

By providing the high refractive index film 6, a light waves 30 incidenton the optical waveguide 2-1 moves from the optical waveguide 2-1 to thehigh refractive index film 6 at an end of the optical waveguide 2-1 andis then introduced into the optical waveguide 2-2 from the other end ofthe high refractive index film 6. The light wave introduced into theoptical waveguide 2-2 is output as output light 31 of the opticalwaveguide device to the outside of the optical waveguide device.

The refractive index of the high refractive index film 6 is higher thanthat of the optical waveguide 2-1 (or 2-2), and the refractive index ofthe bonding layer surrounding the high refractive index film 6 is lowerthan that of the thin plate 1 surrounding the optical waveguide 2-1.Accordingly, the optical waveguides 2-1 and 2-2 can be connected to eachother through the high refractive index film 6 with smaller curvaturethan in a case in which the end of the optical waveguide 2-1 is bent tobe connected to the optical waveguide 2-2. With the configuration shownin FIG. 5, it becomes possible to make an optical integrated circuit, inwhich a light modulator and the like are integrated, compact.

For example, semiconductors, such as Nb₂O₅, TiO₂, Ta₂O₅, As₂S₃, and Si,may be preferably used for the high refractive index film 6 describedabove, and a reactive RF sputtering method may be used when forming thehigh refractive index film 6.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it becomes possible torealize an optical waveguide device having multiple functions or highperformance, to improve the productivity of products, and to provide anoptical waveguide device capable of suppressing deterioration of anoperating characteristic of the optical waveguide device.

1. An optical waveguide device comprising: a thin plate having athickness of 20 μm or less; and at least an optical waveguide formed inthe thin plate, wherein the thin plate is bonded and fixed to asupporting substrate with an adhesive interposed therebetween, and afilm having a higher refractive index than the thin plate is provided ona surface of the thin plate bonded and fixed to the supporting substrateso as to be in contact with a part of the optical waveguide.
 2. Theoptical waveguide device according to claim 1, wherein the film having ahigher refractive index has a strip line shape, and forms an opticalwaveguide having the film having a high refractive index as a core, andat least a part of light waves propagating the optical waveguide formedin the thin plate propagates inside the film having the higherrefractive index and having the strip line shape.
 3. The opticalwaveguide device according to claim 2, wherein an end of the strip lineshape of the film having a high refractive index becomes thick along theoptical waveguide formed in the thin plate.
 4. The optical waveguidedevice according to claim 2, wherein a width of the strip line shape ofthe film having a high refractive index is the same as a width of theoptical waveguide formed in the thin plate.
 5. The optical waveguidedevice according to claim 2, wherein at least a part of light wavespropagating the optical waveguide formed in the thin plate is output asa led-out light wave from an end of the strip line shape of the filmhaving a high refractive index to outside of the optical waveguidedevice.
 6. The optical waveguide device according to claim 3, wherein atleast a part of light waves propagating the optical waveguide formed inthe thin plate is output as a led-out light wave from the end of thestrip line shape of the film having a high refractive index to outsideof the optical waveguide device.
 7. The optical waveguide deviceaccording to claim 4, wherein at least a part of light waves propagatingthe optical waveguide formed in the thin plate is output as a led-outlight wave from an end of the strip line shape of the film having a highrefractive index to outside of the optical waveguide device.
 8. Theoptical waveguide device according to claim 2, wherein each end of thestrip line of the film having a high refractive index is connected todifferent optical waveguides formed in the thin plate which are notconnected continuously to each other, respectively.
 9. The opticalwaveguide device according to claim 3, wherein each end of the stripline of the film having a high refractive index is connected todifferent optical waveguides formed in the thin plate which are notconnected continuously to each other, respectively.
 10. The opticalwaveguide device according to claim 4, wherein each end of the stripline of the film having a high refractive index is connected todifferent optical waveguides formed in the thin plate which are notconnected continuously to each other, respectively.